Recently, digital audio tape recorders have made their appearance in the audio field, while an 8mm-wide tape standard, narrower than the conventional 1/2 inch-wide standard, has appeared in the VTR field, and these have become growingly popularized, so that there has been a growing demand for improving the magnetic recording medium to be capable of making higher-density recordings: i.e., it is essential to materialize a high-efficiency magnetic recording medium having excellent electromagnetic conversion characteristic and running durability.
In order to meet the demand, various investigations have until now been made; for example, regarding a ferromagnetic powder, it is known that the use of one having a larger specific surface area according to a BET method or a smaller particle size enables to improve the electromagnetic coversion characteristic of a magnetic recording medium. However, the magnetic recording medium comprising a ferromagnetic powder with a large BET value of specific surface area has a problem that it is liable to cause edge creases and shedding trouble and is inferior in the running durability.
On the other hand, the use of a ferromagnetic powder with a small BET value improves the running durability but leaves room for further improvement of the electromagnetic conversion characteristic.
Also, an attempt has been made to compose a magnetic layer of a construction of two layers of which the upper layer contains a ferromagnetic powder having a smaller aspect ratio and the lower layer contains a ferromagnetic powder having a larger aspect ratio, as described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 9813/1982.
However, a magnetic recording medium of such a construction as disclosed in this publication improves the electromagnetic conversion characteristic but is insufficient for the upper layer's adhesion because of the too small aspect ratio of the ferromagnetic powder contained in the upper layer, thus being liable to cause shedding and dropout trouble, which leads to a significant deterioration of the running durability. This problem cannot be covered merely by trying to improve the running durability by relatively increasing the aspect ratio of the ferromagnetic powder contained in the lower layer.
Namely, it could not be successful to improve both the electromagnetic conversion characteristic and the running durability of a magnetic recording medium at the same time through the characteristics of the ferromagnetic powder contained in the magnetic layer. It is the present situation that conventional magnetic recording mediums have not attained technical feasibility for well balancing both electromagnetic conversion characteristic and running durability.